From 1885 when Lord Rayleigh first described the acoustic waves that travel along the earth's surface to present day use in scanning and communication systems, scientists have been interested in the action of acoustic waves in solid materials. It was demonstrated several years ago by workers at Bell Telephone Laboratories that ultrasonic waves with frequencies of a billion hertz and up are capable of travelling several centimeters through a solid medium. Best known, of course, for the use of acoustic waves in a solid medium is the piezoelectric crystal. Compression of such a crystal generates an electrostatic voltage across it and, conversely, application of an electric field may cause the crystal to expand or contract in certain directions.
In recent years, the acoustic wave technology expanded rapidly after the developed of the interdigital transducer, an efficient type of transducer for converting an electrical signal into an acoustic surface wave and for reconverting the acoustic wave back into an electrical signal. When such an interdigital transducer is placed on a piezoelectric material such as quartz or lithium niobate (LiNbO.sub.3), and a rapidly changing electrical signal is applied to the transducer, the piezoelectric material will vibrate in unison with the electrical signal, generating a sound wave.
Combining the use of acoustic waves and optical light sources is known as acousto-optics. Acousto-optics is commonly used for achieving modulation and scanning of laser beams. This is normally done by using bulk acoustic waves which produce index of refraction variations within a material. These index of refraction variations interact with a laser beam causing diffraction of the beam. By turning the acoustic wave on and off, the amount of diffraction can be changed, achieving modulation of the diffracted light. By changing the frequency of the acoustic wave, the direction of diffraction can be changed, resulting in scanning of the output beam in angle.
The same type of deflection and modulation has been achieved, as shown in the art, by interaction of the light with acoustic surface waves. Use of surface waves rather than bulk waves is advantageous because attenuation can be lower (resulting in potentially higher resolution from a scanner), acoustic velocity is slower (resulting in smaller devices), and high frequency transducers are easier to fabricate. However, achieving interaction of light with waves is difficult. In the past, it was accomplished by coupling light into a thin film waveguide on the material surface, interacting with the surface wave, and then coupling out of the thin film. The need for such coupling makes devices utilizing thin films on the surface of materials unattractive for systems applications because of light loss in the coupling process and optical quality problems in traversing the thin film. See an article by Yoshiro Ohmachi in the Journal of Applied Physics, Vol. 44, No. 9, September 1973, pps. 3928 to 3933, entitled "Acousto-optical Light Diffraction in Thin Films".
According to the present invention, a technique is described for using acoustic surface waves to produce light deflectors and modulators. The interaction between the light and the acoustic waves is achieved by allowing the light to suffer total internal reflection near grazing incidence from the active device surface in a plane perpendicular to the direction of travel of the acoustic wave. Such devices have possible bandwidth, number of resolvable scanning spots, and cost advantages over currently used bulk wave devices.